Perhaps you just want to remove any patterns that are not allowed...

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11152608
use warnings;

my @lines = split /^/, <<END;
xxx<pre>xxx<www>xxx<strong>xxx
xxx<pre>xxxxxx<strong>xxx
xxx<pre>xxx<pr>xxx<strong>xxx
xxx<pre>xxx<pree>xxx<strong>xxx
xxx<pre>xxx<strong>xxx<strong>xxx
xxx<pre>xxx<pre>xxx<strong>xxx
END

my %allowed = map { ( '<'.$_.'>' ) x 2 } qw( pre strong );
for ( @lines )
  {
  my $clean = s[<\w+>][ $allowed{$&} // '' ]ger;
  print $clean;
  }
[download]

Outputs:

xxx<pre>xxxxxx<strong>xxx
xxx<pre>xxxxxx<strong>xxx
xxx<pre>xxxxxx<strong>xxx
xxx<pre>xxxxxx<strong>xxx
xxx<pre>xxx<strong>xxx<strong>xxx
xxx<pre>xxx<pre>xxx<strong>xxx
[download]